Electrode composition for electron-discharge devices



Feb. 11, 1930. s. DUsHMAN 1,747,062

ELECTRODE COMPOSITION FOR ELECTRON DISCHARGE DEVICES Filed July 2', 1923 MA GNE6/UM OE CAL C/UM Inventor: Saul Dushman,

His Attorney.

'is relatively low and which therefore Patented Feb. 11, 1930 UNITED STATES PATENT OFFICE SAUL DUSHHAN, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK ELECTRODE COMPOSITION FOR ELECTRON-DISCHAtRGE DEVICES Application filed July 2, 1923. Serial No. 648,876.

The present invention relates to the constitution of cathodes for electrical discharge devices, as for example, radio detectors, rectifiers and the like.

The object of my invention is to provide a cathode material capable of high electron emission at an operating temperature which ermits of operation of the device by a ow heating current, such as an ordinary primary battery is capable of furnishing.

I have found that the element of zirconium is capable of a high electron emission at a temperature as low as about 1400 to 1500 degrees centrigrade and I have found that when an oxide of zirconium is incorporated into an electrode which is operated under conditions permittin reduction of at least part of the zirconium that the electron emissivity of the electrode is very materially improved.

lVhen making an electrode embodying my invention, a finely divided zirconium compound is incorporated with the material constituting the main body of the electrode, as distinguished from the surface application of a layer of oxide. For example, in the manufacture of a tungsten cathode embodying my invention, a suitable non-volatile compound of zirconium is mixed with the finely divided metallic tungsten from which cathode filaments are fashioned either in accordance with the Coolidge mechanical working process described in the U. S. Patent 1,082,933 of December 30, 1913, or by squirting of the finely divided material admixed with a uitable binder.

\Vhen manufacturing drawn tungsten cathodes about one to two oercent of zirconia or other suitable non-volatile compound of zirconium is mixed with the finely divided tungsten metal before it is pressed into slugs preparatory to sintering. In general, as much zirconia, Zr.O is added as may be tolerated in the tungsten without interfering with the subsequent swaging and drawing in accordance with Coolidge Patent 1,082,933.

In some cases it may be more convenient to form electrodes from a mixture of metal and a zirconium compound by adding a suitable binder in suflicient quantity to produce a plastic mass and squirting the same into filaments or ribbons. For example, filaments may be made by squirting a mixture comprising by weight about 95 parts tungsten, 5 parts zirconia and enough carbonaceous binderto form a mass capable of being squirted, and thereupon baking, treating to remove the binder, and sintering in the usual way.

l/Vhen electrodes made in this way are mounted as cathodes in an electron discharge device, for example, as shown for example in the drawing, comprising a bulb 1, a cathode 2, an anode 3 and a grid 4, precautions should be taken to thoroughly evacuate the bulb and contained parts, especially to remove water vapor. The electron device is provided as usual with the cathode leads 5, 6, and anode lead 7 and a grid lead 8, all of which are sealed into a stem 9 and connected to terminals (one numbered 10 being shown) sealed into a case 11. However, as these structural features form no essential part of my invention they will not be described in greater detail. Preferably, but not necessarily, a material highly reactive to Water vapor, as for example, magnesium or calcium is introduced into the bulb.

In the device shown in the drawing this maltlerial is indicated as a film on the bulb wa When the evacuation is complete and preferably when the bulb has been sealed the cathode is activated by heating to a temperature of about 2500 degrees centigrade for about a minute and then formed at about 1450 to 1650 degrees centigrade for a few minutes. Heating to 2500 degrees centigrade results in a reduction of a small part of the zirconia to zirconium which diffuses at the forming temperature to the cathode surface where it remains in greater part as an adsorbed film. The temperature of normal operation preferably is materially lower, usually below 1450 degrees centigrade as at the higher temperatures zirconium tends to be vaporized from the incandescent tungsten. At 2500 degrees centigrade the evaporation is quite rapid. In general it may be said that the zirconium-containing cathodes should be operated below bright incandescence, say at a bright red heat.

Cathodes containing zirconia after having been activated and formed as above described, when operated at 1425 degrees centigrade give an electron emission of at least about 30 milliamperes per sq. cm. of emitting surface whereas pure tungsten at the same temperature has the extremely low electron emission of about 0.0006 of a milliampere per sq. cm.

Conveniently, but not necessarily a nonvolatile reducing agent, such as carbon is incorporated With the zirconia into the cathode in order to assist in reducing the zirconia particularly when for any reason water vapor is evolved even though in minute quantity during the operation of the device. Carbon may be introduced by heating the filament in the presence of the vapor of a carbon compound,

as for example, the vapor of anthracene or naphthalene. The pressure of this vapor should be maintained at a low value and the temperature of the cathode, the time of treatment and other conditions should be so regulated as to maintain the carbon content less than about one per cent and preferably as low as about 0.5%.

Although the manufacture of a tungsten base electrode has been described for the purpose of illustrating my invention, I wish it to be understood that the benefits of my invention also may be obtained in the case of electrodes consisting wholly or largely of other materials, as for example, molybdenum or nickel.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. The process of treating an electrode containing zirconium as a constituent which consists in heating said electrode in a non-oxidizing environment to about 2500 degrees centigrade, then heating to about 1450 to 1650 degrees centigrade, and finally operating said electrode at a lower temperature.

2. The process of treating an electrode containing zirconium as an elementary constituent in a non-oxidizing environment which consists in heating said electrode to about 2500 degrees centigrade for about one minute,

then heating to about 1450 to 1650 degrees centigrade for several minutes, and finally operating said electrode at a temperature below about 1450 degrees centigrade.

In witness whereof I have hereunto set my hand this 30th day of June, 1923.

SAUL DUSHMAN. 

